In the anthraquinone cyclic process for the preparation of hydrogen peroxide, one or more alkyl anthraquinones and/or alkyl tetrahydroanthraquinones are used as the reactants, to form hydrogen peroxide in a cyclic process. Reactants are used in the form of a “working solution” comprising one or more water-immiscible solvents.
In the first stage working solution is hydrogenated with hydrogen or a hydrogen-containing gas in the presence of a catalyst. This results in the quinones present being at least partially converted into the hydroquinone form. In a second stage, the working solution is reacted with an oxygen-containing gas, e.g. air, to cause the hydroquinones to form hydrogen peroxide accompanied by the reformation of quinones. In the third stage, the hydrogen peroxide dissolved in the working solution is extracted with water to form an aqueous hydrogen peroxide solution. Finally, the extracted, oxidized working solution (which, in practical terms no longer contains any hydrogen peroxide), is returned to the first, hydrogenation, stage.
During the cyclic process described above, small amounts of by-products are produced from the reactants, preventing them from continuing to form hydrogen peroxide. In the case of working solutions containing alkyl tetrahydroanthraquinones, for example, alkyl tetrahydroanthraquinone epoxides are formed as by-products. In order to minimize the loss of reactant due to by-product formation, the anthraquinone cyclic process may be performed with an additional regeneration stage in which all or part of the by-products are converted back into active reactants.
There have been a number of reports discussing various aspects of the regeneration step. For example, it is known that regeneration of the working solution can be achieved by treatment with alkalizing solids such as: Al2O3 and MgO (U.S. Pat. No. 2,739,875), CaO and Ca(OH)2 (GB 838,939), sodium aluminum silicates (U.S. Pat. No. 3,055,838), and alkali-treated solids (FR 1 305 640). DE 12 73 499 disclosed that regeneration with a basic solid is more effective with a hydrogenated working solution than with an oxidized working solution.
All regeneration processes which utilize a solid regenerant have the disadvantage that acid degradation products contained in the working solution cause the activity of the regenerant to decrease within a short period of time. As a result used regenerant must be replaced with fresh regenerant at frequent intervals and disposed of in a procedure that is both cumbersome and expensive.
It is known from DE-A 11 38 743 that an oxidized and extracted working solution can be regenerated by treatment with oxygen or another oxidizing agent, such as H2O2, in the presence of an aqueous solution of a base.
DE-A 12 03 237 describes how oxidized and extracted working solution can also be regenerated in the absence of an oxidizing agent by treatment with an alkali hydroxide or aqueous lye. This reference indicates that the recycle solution should be treated after full oxidation to prevent dissolution of the hydroquinones in the aqueous phase.
U.S. Pat. No. 2,901,491 describes how the hydroquinones in a hydrogenated working solution are extracted from it in full with sodium hydroxide solution. By oxidizing the aqueous extract with air, the extracted hydroquinones can be converted back into the quinones, which then separate out or can be extracted with a solvent. The working solution extracted with sodium hydroxide solution may be distilled to recover solvents.
FR 1,342,577 describes the regeneration of working solution with a highly alkaline dithionite solution. The reactants contained in the working solution are extracted almost completely from the organic phase with this solution. After phase separation, the organic phase is recovered by distillation. The aqueous phase obtained during the treatment is oxidized, e.g., by passing air through it, thereby causing the reactants to separate out in the form of quinones. The treatment has a regenerative effect since, after treatment, the quantity of quinones obtained is greater than that in the working solution originally used. A disadvantage of the process, however, is that it consumes large amounts of dithionite and requires an additional distillation stage.